Two genetic systems in the mouse have been studied -- the Tla-H(Tla) system and the W system. Tla-H(Tla) System: We have found that the Tla-region (defined as that segment of the 17th chromosome starting immediately to the right of H-2D and ending at some point distal to Tla) is very complex. With the development of new cross-over congenic strains, we have identified and located several histocompatibility loci and at least one new serologically defined locus, Q-2, in this region. Q-2 determines a differentiation alloantigen(s) present on lymph node, spleen, and thymus cells. W System: By cytotoxicity testing on early erythroid cells taken from livers of 14-day-old fetuses, we have identified two erythroid antigens. One is present predominantly on early erythroblasts while the other is present on more differentiated erythroid cells. By comparing the quantities of these antigens on cells from normal and mutant (W/W and Wv/Wv) fetuses, we have found that the mutant cells differ antigenically from normal ones. BIBLIOGRAPHIC REFERENCES: Flaherty, L. and Wachtel, S.S. 1975. H(Tla) System: Identification of Two New Loci, H-31 and H-32, and Alleles. Immunogenetics 2:81-85. Flaherty, L. and Wachtel, S.S> 1975. H(Tla) System: Allelism and Linkage Studies. Transplantation Proc. 7:143-145.